In the past, apparatus have been widely used in providing a unitary package by using a stretched film web to over-wrap a load on a pallet to facilitate long distance or overseas transportation of the palletized packages One of the early types of wrapping apparatus for this purpose is shown in FIG. 3 of the attached drawings, wherein a roll of stretchable film is rotatably disposed with respect to a turntable on which the palletized load is placed so that when the turntable rotates, the film is unwound from the film roll and over-wrapping around the palletized load.
One of the disadvantages of this early type wrapping apparatus is that if the palletized load is rectangular in shape, then the linear speed of the unwound film is unstable owing to the existence of corners of the rectangular load and thus necking or shrinking in lateral dimension may occur during wrapping the load. This may induce a damage to the film web and thus resulting in a poor wrapping.
To address the problem, a later type of stretch wrapping apparatus was developed. The later type apparatus, as shown in FIG. 4, comprises a pair of rollers, an upstream roller and a downstream roller, rotating at different speeds to stretch the film web at a pre-determined amount. Examples of this type of wrapping apparatus are U.S. Pat. Nos. 4,302,920 and 4,387,552.
A disadvantage of this later type wrapping apparatus is that since the rollers are driven by inter-engaged gears, the speed ratio between these two rollers is in generally not adjustable or its adjustment is very limited to certain discrete values. If wrapping with a different degree of stretching is required, the unadjustability of the speed ratio will prevent the wrapping apparatus from wrapping the load with an appropriately-stretched film.
Furthermore, since the diameter of the film roll is decreasing with the consumption of the film, the angle between one of the rollers and the film roll is constantly changed. Similarly, when a rectangular load is wrapped, the angle between the other roller and the load varies from time to time owing to the corners of the rectangular load.
It is therefore desirable to provide a stretch wrapping apparatus which comprises a steplessly adjustable means between the high speed (downstream) roller and the low speed (upstream) roller to provide a stepless speed ratio variation means therebetween.